With commit 945a95d908b4544f7c5dea6caacffdc0f44cffcb SDDM will now support a custom default avatar in the next release. Forgot the link.
Before the patch:
After tha patch:
Note that the above is not quite the same as the logout screen thingy. But I am not sure how to make it render the image in a way that it does not quite touch the edge of the border.
To illustrate what I mean:
With the logout screen the user icon is not touching the circle border.